Type inference for discriminator models are broken

[lobabob]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.16.2

Node.js version

22.13.1

MongoDB server version

8.0.4

Typescript version (if applicable)

5.7.3

Description

Discriminator models only have the _id and __v properties associated with their typings. All type inferences from its schema definition or the schema definition of its parent are lost.

Steps to Reproduce

const ParentSchema = new Schema({
  field1: {
    type: String,
    require: true,
  },
  field2: Number,
}, {
  discriminatorKey: 'field1',
})

const ParentModel = mongoose.model('Parent', ParentSchema)

const ChildSchema = new Schema({
  field3: String,
})

const ChildModel = ParentModel.discriminator('child', ChildSchema)

Typescript shows the fields associated with ChildModel and any documents created via ChildModel are just _id and __v when they should include field1, field2, and field3.

Expected Behavior

Correct type inference should show that fields associated with ChildModel and any documents created via ChildModel should include field1, field2, and field3.

[vkarpov15]

We're working on a fix. As a workaround, you can explicitly type ChildModel as follows:

import mongoose, { InferSchemaType, Schema } from 'mongoose';

const ParentSchema = new Schema({
  field1: {
    type: String,
    require: true,
  },
  field2: Number,
}, {
  discriminatorKey: 'field1',
})

const ParentModel = mongoose.model('Parent', ParentSchema)

const ChildSchema = new Schema({
  field3: String,
})

const ChildModel = ParentModel.discriminator<
  InferSchemaType<typeof ParentSchema> & InferSchemaType<typeof ChildSchema>
>('child', ChildSchema)

const doc = new ChildModel({});
doc.field1;
doc.field2;
doc.field3;

[vkarpov15]

Fix will be in v8.17, you'll be able to do the following:

import mongoose, { InferSchemaType, Schema } from 'mongoose';

const ParentSchema = new Schema({
  field1: {
    type: String,
    require: true,
  },
  field2: Number,
}, {
  discriminatorKey: 'field1',
})

const ParentModel = mongoose.model('Parent', ParentSchema)

const ChildSchema = new Schema({
  field3: String,
})

const ChildModel = ParentModel.discriminator('child', ChildSchema)

const doc = new ChildModel({});
doc.field1;
doc.field2;
doc.field3;